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L. ESPENSCHIED AND H. A. AFFEL.

MEANS FOR SIGNALING OVER MULTIPLEX TRANSMISSION CHANNELS.

APPLICATION FILED JULY 29,1919.

Patented Sept. 5, 1922.

Patented Sept. 5, 1922.

UNITED STATES 1,428,155 PATENT OFFICE.

LLOYD ESPENSCHIED, OF HOLLIS, AND HERMAN A. AFFEL, OF BROOKLYN, NEW YORK,

ASSIGNORS TO AMERICAN TELEPHONE AN D TELEGRAPH COMPANY, A CORPORA- TION OF NEW YORK.

MEANS FOR SIGNALING OVER IYI'ULTIP'LEX TRANSMISSION CHANNELS.

Application filed July 29, 1919. Serial No. 313,993.

To all whom it may concern:

Be it known that we, LLOYD EsrnN SGHIED and HERMAN A. ArrnL, residing at Hollis and Brooklyn, in the counties of Queens and Kings, respectively, and State of New York, have invented certain Improvements in Means for Signaling Over Multiplex Transmission Channels, of which the following'is a specification.

This invention relates to transmission systems and more particularly to multiplex transmission systems employing carrier currents. y

Where multiplex transmission is attained by the use of carrier currents a plurality of carrier frequencies are transmitted over acommon transmission line, the carrier frequencies being separated sufficiently to prevent interference and each frequency be ing assigned for use as a separate channel of communication so that signals, such, for instance, as telephonic variations may be transmitted by impressing them upon the carrier frequencies. Several channels may thus be superposed upon a common transmission line and each channel performs the functions of an ordinary telephone transmission line.

In ordinary telephone transmission it is customary to transmit overthe line, in addition to the. telephone currents, ringing currents or other signaling currents for the purpose of signaling between operators in establishing connections. In order'to obtain practical telephone service over each carrier communication channel it is desir able that the channel be utilized for the transmission of such ringing or other signaling currents, as well. as for the transmis sion of the ordinary telephone currents.

One feature of this invention'is the pro vision of means,'whereby ringing and other signals may be transmitted for each carrier channel. Another, is the provision of means, whereby each carrier channel itself may be utilized for the transmission of the ringing or other signaling currents neces sary in the establishment of, communication over the channel. The invention further provides means, whereby the ringing or other signaling currents to be transmitted over a telephone" carrier channel. may be used to modulate the carrier frequency a d to that channel in a manner similar to that employed in carrier telephone transmission. The invention also comprises certain other features that will become manifest as the description thereof proceeds. These several features of the invention and their manner of operation may now be understood from the following description, when read in connectionuwith the accompanying drawing, which constitutes a circuit diagram of one embodiment of-v the said invention. i r l 1 Referring to the drawing, ML designates a transmission line employed for the trans mission of a plurality ofcarrierfrequencies, said transmission line being associated through a balanced transformer arrangement 10, with a common transmitting circuit TL, upon which. the carrier. frequencies from the severaltransmitting channels are impressed, and a common receiving circuit BL from which the several received carrier frequenciesare distributed to corresponding receiving channels. In order that the common circuits TL and RL may be conjugate with respect to each other, that is, may be. so arranged that electrical variations arising in one circuit shall not affect the other, an artificial line or'networlr MN is provided for. balancing the main line ML. Thecommon circuit This associated for transmitting purposes. with each of a pluralityof low frequency transmission lines through carrier transmitting channels, such as TL, and TL associated with line terminals L and L In a similar manner the receiving circuit RL islassociated with said low frequency lines through receiving carrier channels such as EL and RL,.

In order to translate the low frequency currents, such as telephone currents transmitted over the line terminals :L and I1, into high frequency carrier currents for transmission over the line ML, the transmitting channels TL' TL etc., are provided with modulating arrangements, such as M and M These modulating arrangeients may be of any desiredcharacter, but are preferably duplex vacuum tube modulators of the character illustrated in the application of 13. IV. Kendall, SerialNo. 125,3-i9, filed October 13, 1916, or in the patent to John R. Carson, No. 1,348,307,

dated June 15,1920. -As set forth in said applications these modulators are so arranged that normally the carrier oscillations are suppressed, but when low frequency signaling currents, such as telephone currents are impressed thereon, carrier oscillations are transmitted, having an amplitude directly proportional to the modulating current.

In addition to the modulating arrangement the transmitting channelTL includes a band filter TF an amplifier TA, and a band filter TF The band filters TF and TF, are of the general type disclosed in U. S. Patents to George A. Campbell, Nos. 1,227,113 and 1,227,114, dated May 22, 1917. These filters are preferably so designed as to transmit a band of frequencies extending either above or below the carrier frequency assigned to the channel by an amount equal to the range of frequencies employed in ordinary telephonic transmission. The amplifier TA may be of any well-known character, but is preferably an amplifier of the well known vacuum tube type. In a similar manner the transmitting channel TL is provided with a filter TF an amplifier TA and a filter TF These filterslikewise are of the type disclosed in the Campbell patents above referred to and are designed to transmit a similar range of frequencies with respect to the carrier frequency assigned to this channel. This carrier frequency should be sufficiently separated from that of the channel TL in the broad frequency scale so that interference is substantially pre vented.

In order to translate received carrier frequencies into low frequency signaling currents for transmission to the line terminals L and L the carrier receiving channels BL, and BL, are provided with demodulating or detecting arrangements, such as D, and D These demodulator or detecting arrangements may be of any well known type, but are preferably of the duplex vacuum tube type illustrated and described in the patent to John E. Carson, No. 1,343,308, dated June 15, 1920. The detecting or modulating arrangement of the said patent of John E. Carson employs the so-called homodyne method of detecting, thereby necessitating that oscillations of the same frequency as the carrier oscillations supplied to the corresponding modulator be also sup plied to the demodulator.

In addition to the demodulating arrangement the receiving channel RL includes a filter RF an amplifier HA and a filter RE. The filter RF, may be similar to the filters TF and TF, of the corresponding transmitting channel. The filter RF, is also of the general type disclosed in the patents to Campbell above referred to, but is so designed as to transmit low frequency currents extending overthe range employed in ordinary telephone transmission. The amplifier RA, may be of any well-known character, but is preferably ofthe well known vacuum tube type and is so arranged as. to efficiently amplify low frequency currents. In a similar manner the receiving channel BL is provided with a filter RIT an amplifier RA and a filter RF The filter RF, is similar to the filters TF and TF, of the corresponding transmitting channel, while the filter RF is similar to the filter RF, already described. The amplifier RA, is likewise similar to the amplifier 13A,.

In order to supply carrier oscillations to the modulators and demodulators a circuit 13 is provided upon which is impressed the several carrier frequencies. These frequencies may be generated in any well-lmo'wn manner, but are preferably supplied by gen erating a fundamental frequency and then producing harmonics thereof in accordance with the frequency control system disclosed in the British Patent No. 131, 126, llovember 13, 1919. The carrier frequency assigned to the transmitting and receiving channels of the line terminals L may be selectively transmitted from the circuit 18, through a filtering arrangement CF which may be a simple tuned circuit as illustrated. The desired frequency is then supplied to the modulator M and the demodulator l) through branch circuits 14 and 15, respectively. Similarly, the carrier frequency assigned to the transmitting and receiving channels of the line terminals L may be transmitted through a tuned circuit CI from which it is supplied to the modulator M and demodulator D through branch circuits 16 and 17.

The transmitting and receiving channels TL and BL, are associated with the low frequency line terminals L through a balanced transformer arrangement 11 and are rendered conjugate with respect to each other by balancing the line associated with terminals L by an artificial line or network N In a similar manner the transmitting and receiving channels TL and EL, are as sociated with low frequency line terminals L through a balanced transformer 12 and are rendered conjugate with respect to each other by balancing the line associated with terminals L by an artificial line or network N In order to associate the line terminals L, and L with various terminal circuits, said means are arranged to terminate in jacks J and J respectively.

In order that a ringing signal may be transmitted from the line terminal L a key K is associated therewith, said key controlling a relay 18, whereby ringing current may be supplied through the transformer 11 to the modulator M In a similar manner a key K is associated with the line terminals L, and controls a relay 19, whereby ringing current may be supplied throughthe trans- In order including this relay'being' so tuned as tobe responsive to currents of ringing frequency but to afford a high impedance to voice currents. The relay 20 is arranged to control through a relay 21 a lamp or other signal S associated with the line terminals L Similarly, a ringing responsive relay 22 is bridged across the line terminals L [and controls a lamp or other signal S through a relay 23.

The apparatus so far disclosed is characteristic of the terminal arrangements at'one end of the main transmission" line "ML and it will be understood that similar arrangements may be provided at the distant termi nal of the main line. Furthermore, while receiving and transmitting channels have only been illustrated for two line terminals such as L and L5, it will be understood that similar channels for other low frequency lines and having other characteristic carrier frequencies assigned thereto may be associated with the common transmitting and receiving circuit TL and RL.

y be clear from a description of the operation Further details of the apparatus will now which is as follows: 1 Talking currents incoming from the line terminals L are transmitted over the back contacts of relay 18 and through the transformer 11 to the input circuit of the modulator M Normally, high frequency oscillations from the circuit 14; are impressed on the common branch ofthe input circuit of the modulator M but due to the balanced arrangement offthe modulator circuit, these oscillations are not transmitted to the filter TF Talking currents impressed upon the 'input circuit of the modulator, however,

unbalance the modulator circuit so that high frequency oscillations modulated in direct proportion to the talking currents are transmitted from the output circuit of the modu-. lator M,, through the filter TF to the am plifier TA Upon being amplified by said amplifier the modulated oscillations are transmitted through the filter TF, to the common transmitting circuit TL. These modulated high frequency oscillations, together with the modulated high frequency oscillations from other transmitting channels are impressed through the transformer 10 upon the main transmission line ML, over which they are transmitted. to similar carrier apparatus at a distant station.

. Modulated oscillations of the several carrier fre uencies incoming from the line ML tt l mitted to the common receiving modulator 1),. These modulated oscillations react with unmodulated carrier oscillations from the c1rcu1t15, with the result that currents simulating the low frequency tele-,

phone currents, in accordance with which the received oscillations were modulated, ap pear in the output circuit of the demodulator. The low frequency telephone currents are then amplified by the amplifier RA, and transmitted through the low frequency filter RF to the midpoints of windings of transformer 11, from which point they are transmitted to the artificial line M and over the back contacts of relay 18 to the 'lineterminals L 1 a If it is desired to transmit a ringing signal from the line terminals L to the distant station the key K, may be actuated, thereby energizing the relay 18 so that ringing current is transmitted over the front contacts of said relay and through the transformer 11- of the modulator M in the same manner as in the case of telephone currents. The oscillations modulated in accordance with these ringing currents are then transmitted to the distant station in the same manner as that already described. High frequency oscillations modulated inaccordance with ringing currents at a distant station and incoming from the line ML are transmitted over the tircuit EL and through the filter RF, to the input'circuit of the demodulator D The demodulator detects the ringing currents in accordance with which the carrier oscilla tions were modulated and impresses them upon the amplifier RA, Upon being amplified they pass through the filter RF, to the midpoints of the windings of the transformer 11. From this point the ringing current passes in one direction to the artificial line N and in the other direction over the back contacts of relay 18 to the ringing responsive relay 20, the bridge circuit in which this relay is included being tuned to and need not be further considered.

It will beseen that by means of the arrangements above described, ringing or other signals may be transmitted over the carrier channels of a multiplex system without interference and without necessitating any substantial change in the high frequency apparatus of the carrier system. It will also be obvious that While the invention has been illustrated as embodied in a circuit arrangement, which is deemed quite desirable from the standpoints of efiiciency and simplicity, the general principles herein disclosed may be embodied in many other organizations widely different from those illustrated without departing from the spirit of the invention as defined in the following claims.

IN hat is claimed is:

1. In a signaling system, a circuit for the transmission of high frequency carrier currents, a circuit for the transmission of low frequency currents such as telephone currents, a two-way connection between said circuits comprising two paths, one for transmission in one direction and the other for transmission in the opposite direction, a modulating repeater in one of said paths, a demodulating repeater in the other path, meansto supply carrier current for said modulating and demodulating repeaters, means whereby telephone currents may be transmitted from said second mentioned circuit over one of said paths to said modulating repeater, means whereby ringing current may be transmitted over said path to said modulating repeater, said modulating repeater functioning to supply to said first mentioned circuit carrier currents modulated in accordance with said telephone or ringing currents, and means to transmit carrier currents modulated in accordance with telephone or ringing currents from said first mentioned circuit to said demodulating repeater, said repeater functioning in response thereto to supply to said second mentioned circuit the telephone or ringing currents in accordance with which said carrier currents were modulated.

2. In a signaling system, a circuit for the transmission of high frequency carrier currents, a circuit for the transmission of low frequency currents such as telephone currents, a two-way connection between said circuits comprising two paths, one for transmission in one direction and the other for transmission in the opposite direction, a modulating repeater in one of said paths, means to supply carrier current for said modulating repeater, means whereby telephone currents may be transmitted from said second mentioned circuit over said path including said modulating repeater to said repeater, and means whereby ringing current may be transmitted over said path to said repeater, whereby said modulating repeater functions to supply to said-first mentioned circuit carrier currents modulated in accordance with said telephone or ringing currents.

3. In a signaling system, a circuit for the transmission of high frequency carrier currents, a circuit for the transmission of low frequency currents such as telephone currents, a twoway connection between said circuits comprising two paths, one for transmission in one direction and the other for transmission in the opposite direction, a demodulating repeater in one of said paths, means to transmit carrier currents modulated in accordance with telephone currents and carrier currents modulated in accordance with ringing currents from said first mentioned circuit to said demodulating repeater, whereby the latter supplies to the path in which it is included the telephone or ringing currents in accordance with which the carrier currents were modulated, and means to transmit said telephone or ringing currents over said path to said second mentioned circuit.

4:. In a signaling system, a pair of intercommunicating stations, a signaling channel at one station, a corresponding signaling channel at the other station, a demodulator in said second mentioned channel, means including a modulator to which telephone currents are applied for supplying high fre quency oscillations modulated in accordance with telephone currents for transmission to the distant station, means .to apply ringing currents" to said modulator whereby high frequency oscillations modulated in accordance with said ringing currents may be supplied by said modulator for transmission to the distant station, and means to impress the oscillations modulated in accordance with said telephone or ringing currents upon the demodulator in the corresponding channel at the distant station whereby said dc modulator translates the modulated oscillations into the telephone or ringing currents in accordance with which said oscillations were modulated.

5. In a signaling system, a pair of intercommunicating, stations, a signaling channel at one station, a corresponding signaling channel at the other station, means including a modulator in saidv first mentioned channel to which telephone currents are applied for supplying high frequency oscillations modulated in accordance with telephone currents for transmission to the cor responding channel at the distant station, and means to apply ringing currents to said modulator, whereby high frequency oscillations modulated in accordance with said ringing currentsmay be supplied by said modulator for transmission to the corre sponding channel at said distant station.

6. In a signaling system, a pair of intercommunicating stations, a signaling channel at one station, a corresponding signaling chanml at the other station, a demodulator in said second mentioned channel, and means whereby high frequency oscillations modulated in accordanceiwith telephone currents and high frequency oscillations modulated in accordance with ringing currents may be transmitted from said first mentioned channel to the distant station'and applied to said demodulator, said demodulator: functioning to translate said high frequencyoscillations into the telephone currents or ringing currents in accordance with which sa'idoscilla-v tions were modulated.

7. In a signaling system, a circuit for the transmission of high frequencycarrier currents, a plurality of circuits for the transmission of low frequency currents such as telephone currents, a two-way connection between each of said last mentioned circuits and said first mentioned circuit, each con- IIGCtlOIl comprising two paths, one for trans misison in one direction and the other for transmission in the opposite direction, a modulating repeater in one path of each pair, a demodulating repeater in the other path of each pair, means to supply carrier current for said modulating and demodulating repeaters, means whereby telephone currents may be transmitted from each of said plurality of circuits over one path of each pair to the corresponding modulating repeater, meansjwhereby ringing current may 'be transmitted over each of said paths to said modulating repeaters, said modulating repeaters functioning to supply to said first mentioned circuit carrier currents modulated in accordance with said telephone or ringing currents, and means to transmit a plurality of carrier currents modulated in ac cordance with telephone or ringing currents from said first mentioned circuit andover the other paths of each pair to said demodulating repeaters, said repeaters functioning in response thereto to supply toeach of said plurality of circuits the telephone or ringing currents in accordance with which said carrier currents were modulated,

8. In a signaling system, a circuit for the transmission of high frequency carrier currents, a plurality of circuits for the trans mission of low frequency currents such as telephone currents, a two-Way connection between each of said plurality of circuits and said first mentioned circuit, each twoway connection comprising two paths, one for transmission in one direction and-the other for transmission in the opposite direction, a modulating repeater in one path of each pair,means to supply carrier currents for saidmodulating repeaters, means whereby telephone currents may be transmitted from each of said plurality of circuits over said paths including modulating repeaters to each of said, repeaters, and means whereby ringing current may be transmitted over each of said paths 'to said repeaters, whereby said modulating repeaters function to supply to said first mentioned circuit carrier currents modulated in accordance with said telephone or ringing currents. i v

9. In a signaling system, acircuit for the transmissionof high frequency carrier currents, a plurality of circuits for the transmission of low frequency currents such as telephone currents, a two-way connection between each of said plurality of circuitsand,

rier currents were modulated, and means to transmit said telephone or ringing currents over said paths to said plurality of circuits.

10. In a signaling system, a pair of intercommunicating stations, a plurality of signaling channels at one station, a plurality of correspondingsignaling channels at the other station, a demodulator in each of said second mentioned channels, means including a modulator in each of said first mentioned channels to which carrier currents are applied for supplying high frequency oscillations modulated in accordance with telephone currents for transmission to the distant station, means to apply ringing currents to said modulators whereby high frequency oscillations modulated in accordance with said ringing currents may be supplied by said modulators for transmission to the distant station, and means to impress the several oscillations modulated in accordance with said telephone or ringing currents upon the several demodulators in the corresponding channels at the distant which telephone currents. are applied for supplying high frequency oscillations modulated in accordance with telephone ourrentsfor transmission to the corresponding channels at the distant stations, and means to apply ringing currents to each of said modulators, whereby high frequency oscillations modulated in accordance with said ringing currents may be supplied by said modulator for transmission to the corre sponding channels at said distant station.

12. In a signaling system, a pair of intercommunicating stations, a plurality of signaling channels at one station, a plurality of corresponding signaling channels at the other station, a demodulator in each of said second mentioned channels, and means whereby high frequency oscillations modulated in accordance with telephone currents and high frequency oscillations modulated in accordance with ringing currents may be transmitted from each of said first mentioned channels to the distant station and applied to the corresponding demodulators, said demodulators functioning to translate said high frequency oscillations into the telephone currents or ringing currents in accordance with which said oscillations were modulated.

In testimony whereof we have signed our 25 LLOYD ESPENSCHIED. HERMAN A. AFFEL. 

